The present invention relates to an improved process for the preparation of ascorbic and erythorbic acids. Ascorbic acid, or Vitamin C, is required in the human diet and is widely employed both in tablet form and as an additive in other foodstuffs to meet this need. Erythorbic acid, or isoascorbic acid, is useful as an antioxidant for use in foodstuffs.
Ascorbic acid has most commonly been prepared by a multi-step synthesis from D-glucose via sorbose and 2-ketogulonic acid, many improvements having been made to the original method of Reichstein and Grussner, Helv. Chim. Acta., 17, 311 (1934). Copending U.S. patent application Ser. No. 843,946 filed Oct. 20, 1977 describes a new route to ascorbic acid and erythorbic acid comprising the alkali metal borohydride reduction of 2,5-diketogluconic acid, or lower alkyl esters or salts of this acid, to form a mixture of 2-ketogulonic acid and 2-ketogluconic acid, or the corresponding esters or salts of these acids. The 2-ketogulonic acid is a precursor for ascorbic acid, while 2-ketogluconic acid is a precursor for erythorbic acid. The 2-keto acids are readily converted to the desired ascorbic and erythorbic acids by methods known in the art, for example by base catalyzed cyclization of the corresponding lower alkyl esters.
The borohydride reduction of 2,5-diketogluconic acid results in the presence of borate impurities in the reaction product containing the 2-keto acids. In accord with the present invention, it has now been discovered that the presence of such borate impurities in the 2-keto acid-containing reaction product results in lower yields of ascorbic and erythorbic acids when the corresponding alkyl esters are heated with a base to effect the cyclization to the desired final products.